Vacuum tube



July 17, 1928.

E. Y. ROBINSON vAcuuu TUBE Filed men 19, 1926 J5 1 12106122 07" [7/1252 yzwfim/PMz/vm mmmmm we $640k Will asses:

By illorney:

Patented July 17, 1928;

UNITED STATES PATENT OFFICE.

ERNEST YEom nonmson, or Linux, ENGLAND, assrenoa-ro n'moromurvroxnas ELECTRICAL conrm LIMITED, A comrm'x or REAT BRITAIN.

VACUUM TUBE.

Application filed March 19, 1936, Serial No. 96,050, and in Great Britain December 0, 1981.

This invention relates to vacuum tube devices having electron emitting cathodes therein, such for example as the so-called thermionic rectifiers and valves used in wireless telegraphy and telephony and has for its object to provide an improved construction and arrangement of vacuum tube in which ionization is eliminated or reduced to a minimum in a simple and efiective manner.

The present application is a continuation in part of my application Serial No. 603,715,

filed Nov. 27, 1922, wherein is described and claimed an invention for a particular method of reducing ionization in a vacuum electric tube and constructions for carrying out the method, by reducing the electron path between the electrodes and at the same time confining the electrons to the inter-electrode space whereby all the electrons must travel in short paths only, a vacuum electric tube containing an ionizable rarefied gas or vapour being thus caused to exhibit hard characteristics.

The present invention concerns a particular method of, and means for, confining the electrons in a vacuum electric discharge tube containing such an ionizable rarefied atmosphere, namely, by reducing the activity for the emission of electrons of those portions of the cathode from which electron currents would normally tend to pass outside the inter-electrode space.

The accompanying drawings illustrate my invention.

Fig.1 is a sectional view of the electrodes of a diode arranged in accordance with the invention, and contains curves showing the distribution of temperature and electron emission along the cathode.

Fig. 2 is a similar sectional view of the electrodes of a triode.

Fig. 3 illustrates a practical electrode structure of a diode, with a portion of the anode removed, and

Fig. 4 is a section on the line IV-IV of Fig. 3.

Fig. 5 is a front elevation of a triode of relatively small size such as used for radio reception.

Fig. 6 is a side elevation of the device shown in Fig. 5.

Fig. 7 is a section on the line VII-VII of Fig. 5, but to a larger scale, and

Fig. '8 is a fragmentary view to a still larger scale of the filament tensioning and space current confining means at the lower end of the electrodes shown in Figs. 5 and 6.

Fig. 9 is a sectional view of a difierent form of diode in which the cathode is of the thoriated type heated by an auxiliary cathode or member.

The invention requires the use of what is termed a concentrated source of electrons, by which is meant a cathode which is adapted to be heated and which comprises or approximates to a continuous or unbroken electron-emitting surface. In other words, the cathode is of such construction that when an anode is closely related to it the electrons can only travel in short paths within the inter-electrode space, that is to say, the electrons are prevented from travelling in long paths therein. The invention comprises a particular method of, and arrangement for, reducing ionization in a valve, whereby hard characteristics may be given to a valve which would otherwise have soft characteristics. By a valve having hard characteristics is meant one in which the effects of ionization are reduced to such an extent that the cathode has a much longer life than in the case Of a soft valve. The life of the cathode is determined by the removal of its emitting surface by volatilization owing to its elevated temperature and cathode sputtering by positive ions produced by the electron current. In the case of a valve having a homogeneous filament, such as a tungsten filament. much more ionization can be permitted than is the case in a valve having a heterogeneous filament, such as a thoriated tungsten filament, in which the active thorium surface is only one molecule deep. In addition, in the case of a valve where the space current is controlled by additional means, such as the grid in a three electrode valve, then if the cathode is such that it has a reasonable life in spite of ionic bombardment a further characteristic of a hard valve is that the space current is controllable much more, and to much higher anode voltages by the grid than is the case in a soft valve.

The invention comprises a method of and means for reducing ionization in a valve already having hard characteristics or of giving hard characteristics to a valve which contains an ionizable rarefied atmosphere, by which is meant a valve which has such a gas or vapour pressure therein that in the absence of the ap lication of the method thereto it would exliibit soft characteristics, other conditions being similar. The method permits an increase in the electron emission or alternatively economy in the process of exhausting to the necessary high vacuum. In fact an economy in the exhausting rocess may be efiected in any type of har valve having the invention applied thereto.

Referring now to Fig. 1 of the drawings,

this shows a straight filament cathode 11 and a tubular anode 12 preferably substantially cylindrical and of small diameter. The cathode comprises a concentrated source of electrons so that these can only travel within the gap in paths which are all short. The cathode 11 is shown attached to two co-axial lead-in wires 13 and is only so much longer than the anode 12 that the ends of the cathode are cooled by the wires 13 to such an extent that the electron emission from said end portions is small, or even negligible compared with that from the main or central portion of the cathode. To obtain this result it is necessary that the ends of the cathode which are exposed at the ends of the anode shall be comparatively short, particularly if the supports 13 are relatively thin. It. will be clear, however, that if the lead-in wires or supports 13 are relatively thick and conduct heat away rapidly from the ends of the filament the exposed portions thereof may be of greater length.

.In Fig. 1 the temperature distribution along the filament is typically illustrated by the curve 14, wherein the distances measured at right angles to the filament 11 indicate temperature. The corresponding electron emission curve is indicated at 15. It will be noticed that the middle portions of the curves 14 and 15 are substantially fiat. It will be also noticed that the electron emission from the end portions of the filament is negligible. Said end portions therefore act as electrostatic screens for confining the space current to the small interelectrode space between the filament 11 and the cathode 12. In certain cases the lead-in wires 13 will provide additional electrostatic screenmg.

In the structure shown in Fig. 1 the gap between the cathode and the anode is of the order of 1 mm. but it will be understood that the invention is not limited to gaps of this length as the length of the gap depends on a number of variables, as will be well understood by those skilled in the art.

In Fig. 2 a straight filament cathode 16 is attached to the lead-in wires 17 and surrounded by a grid 18 which preferably comprises a helix of fine wire. The anode 19 comprises a tube which closely surrounds the grid 18 and is of approximately the same length as the latter. A particular feature 'of the arrangement illustrated in Fig. 2 consists in that the lead-in wires 17 actual] extend into the ends of the grid and anod e whereby the electrons are very effectively confined to the short inter-electrode gap which in the present instance may be of the order of 2 mm. but is preferably shorter. This arrangement is notably useful in the case of dull emittin filaments owing to the desirability for re ucing ionization to a minimum even when the vacuum is already extremely high.

Figs. 3 and 4 illustrate a practical form of comparatively large power diode in which the cathode comprises two parallel filaments 20 located in an anode 21 having two tunnels 22 therein through which the filaments 20 pass.

The anode and cathode in the structure shown in Fig. 3 are mounted upon a rectangular framework 23 comprising two parallel side members 24 of insulating material, such as steatite or hard glass and metallic end members 25 and 26. The end member 26 constitutes also a clamp by means of which the structure can be secured upon a -re-entrant tube 27 to the envelope (not shown) of the device. The filaments 20 at their lower ends are secured respectively to metallic members 28 which are mounted upon the side members 24 of the framework and insulated from one another. The filament heating current is led to the members 28 through lead-in wires 29 which pass through the pinch or seal at the end of the re-entrant tube 27 in the well-known manner.

The upper ends of the filaments 20 are secured to a cross strip 30 to which is attached a spring 31 secured in turn at 32 to member 25 of the framework. The filaments 20 pass through guide holes in a metallic cross member 33 secured to the framework as described and claimed in the specification of my co-pending application, Serial No. 717,327, filed June 2, 1924.

The anode comprises two similar metal plates each having two semi-circular channels extending along them so that when superposed said plates provide the circular tunnels 22 previously mentioned, the construction being clearly shown by the aid of Fig. 4. The anode 22 thus constituted is held -in the framework by means of three cross pieces 34 secured to the side members 24 of the framework. The general construction of the framework and the mounting of the electrodes therein is described more fully and claimed in the specification of my appli cation Serial No. 722,811, filed June 27, 1924.

The ends of the filaments 20 at their lower ends are maintained cool by reason of the conduction of heat therefrom by the cross pieces 28, to which said filaments are attached. Across the upper ends of the filaments 20 is attached a metallic strip 35 which serves to maintain these ends of the filaments cool. A valve having the electrode structure illustrated by Figs. 3 and 4 operates in a manner similar to that described with reference to Fig. 1.

The small triode of the radio reception type illustrated in Figs. 5 to 8 inclusive comprises in general a tubular anode 36, a.

straight filament cathode 37 and a helical grid 38. The anode 36 comprises, as clearly shown in Fig. 7, a flattened tube which is attached conveniently by welding to a supportingrod 39 which is held within the pinch or seal upon the re-entrant tube 40 of the valve envelope 41, being attached to one of the lead-in wires 42. The rod 39 extends upwards beyond the end of the anode and is secured to a glass bead 43.

The grid 38 comprises a helix of fine wire which is secured to a V-bloc support 44 as described and claimed in the specification of co-pending applications. Serial No. 752,480, filed November 26, 1924. The said support 44 is secured to a rod 45 which is also held in the pinch 40 and connected to another of the lead-in wires 42. The upper end of the rod 45 extends upwards and is secured to the glass bead 43.

The cathode which is of the dull or bright emitter type is welded at its upper end to a support 46 whichis attached to or integral with a member 47 which is secured in the glass bead 43. The member 47 is connected with a conductor 48 which passes outside the electrodes as clearly shown in Fig. 6 and is carried in the pinch or seal 40 and connected with another of the lead-in wires 42. The lower end of the filament 37 is welded to a member 49 which in turn is attached to a leaf spring 50 disposed nearly at right angles to the filament. Said leaf spring 50 is attached as clearly shown in Fig. 8 to a member 51 which extends laterally from and may be integral with a lead-in wire 52 also secured with a pinch or seal 40 and connected with the last of the four lead-in wires 42. The leaf spring 50 main tains the filament in tension and constrains it to move substantially in an axial direction, or in other words, registers the filament against lateral movement with respect to the other electrodes.

The flared portion of the grid support 44, the latter being generally always at negative potential, serves to confine the electrons to the short inter-electrode space at the left hand end of the anode tube 36 (Fig. 7). The members 46 and 49 maintain the ends of the filament cool and may even extend into the grid as shown in Fig. 8, whereby the space current is more effectively confined to the inter-electrode space.

Fig. 9 shows the electrode structure of a valve in which the cathode is heated by electron bombardment or radiation of heat from an auxiliary cathode or member, said cathode, that is, the main cathode, being of the thoriated type.

The anode is indicated at 53 and the main cathode at 54, these electrodes being in the form of co-axial cylinders with a short interelectrode gap. The main cathode 54 has a thoriated surface as indicated by the heavier line terminating at 55 some distance from each end. The auxiliary cathode or heating member comprises a'helix of wire indicated at 56. The heating current for said auxiliary cathode or member 56 is supplied by means of the lead-in wires 57 and 58. Screens 59 and 60 are provided for preventing any electrons from the helix 56 from travelling to the anode 53. The main space current between the anode 53 and the main cathode 54 is confined to the inter-electrode space by reason of the fact that the ends of the main cathode are not thoriated, that is, not covered with a thorium layer. In prac tice the whole of the outer surface of the cathode may be originally thoriated, in which case as soon as the valve is operated the electrons from the ends of said cathode will travel in long paths and produce ionization, by which the thoriation will be removed from the ends of the cathode, the desired shielding effect being thereafter obtained.

I claim as my invention 1. The method of reducing ionization in a vacuum electric discharge tube which consists ,in producing a flow of electrons from a cathode to an anode in the presence of an ionizable atmosphere all in short paths with in an inter-electrode gap which is sufiiciently short to prevent ionization of said atmosphere which atmosphere in a longer gap would produce gaseous ionization, and producing a sufliciently reduced electron emission from the boundaries of the cathode to substantially completely confine the discharge to said inter-electrode gap.

2. A vacuum electric discharge tube comprising an envelope containing an anode, a thermionic electron-emitting cathode spaced therefrom, and an ionizable rarified atmosphere, the inter-electrode space being sufficiently short to prevent ionization of said atmosphere which atmosphere in a longer gap would produce gaseous ionization, the cathode having means for producing a sufficiently reduced electron emission from the boundaries thereof exposed to the space outside of the inter-electrode space to substantially completely confine the electron emission to said inter-electrode space.

3. A vacuum electric discharge tube comprising an envelope containing an anode, a thermionic electron-emitting filament cathode spaced therefrom within said anode, and an ionizable rarified atmosphere, the space between said electrodes being sufliciently tween said electrodes being sufiiciently short to prevent ionization of said atmosphere which atmosphere in a longer gap would produce gaseous ionization, and supports for the ends of said cathode which have a lower resistance per unit of length than the cathode and are operative to cool the ends of the cathode which lie within the anode suffiiciently to render them substantially nonelectron emitting.

In testimony whereof I have hereunto subscribed by name this second day of March, 1926.

ERNEST YEOMAN ROBINSON, 

